As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes—in one generation, orange might be predominant, and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for. Finally, when blue males become common, orange males will once again be favored. Males and females of certain species are often quite different from one another in ways beyond the reproductive organs.
Such differences are known as sexual dimorphisms Figure 3 , which arise from the fact that in many populations, particularly animal populations, there is more variance in the reproductive success of the males than there is of the females. That is, some males—often the bigger, stronger, or more decorated males—get the vast majority of the total matings, while others receive none. This can occur because the males are better at fighting off other males, or because females will choose to mate with the bigger or more decorated males.
Females, on the other hand, tend to get a handful of selected matings; therefore, they are more likely to select more desirable males. Sexual dimorphism varies widely among species, of course, and some species are even sex-role reversed.
In such cases, females tend to have a greater variance in their reproductive success than males and are correspondingly selected for the bigger body size and elaborate traits usually characteristic of males.
Figure 3. Sexual dimorphism is observed in a peacocks and peahens, b Argiope appensa spiders the female spider is the large one , and in c wood ducks. While it is beautiful and the male with the largest, most colorful tail is more likely to win the female, it is not the most practical appendage. In addition to being more visible to predators, it makes the males slower in their attempted escapes.
There is some evidence that this risk, in fact, is why females like the big tails in the first place. The speculation is that large tails carry risk, and only the best males survive that risk: the bigger the tail, the more fit the male. This idea is known as the handicap principle. The good genes hypothesis states that males develop these impressive ornaments to show off their efficient metabolism or their ability to fight disease. Females then choose males with the most impressive traits because it signals their genetic superiority, which they will then pass on to their offspring.
Though it might be argued that females should not be picky because it will likely reduce their number of offspring, if better males father more fit offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many, weaker offspring.
In , biologist Ronald Fisher proposed another model of sexual selection: the Fisherian runaway model , which suggests that selection of certain traits is a result of sexual preference.
Natural selection is a driving force in evolution and can generate populations that are better adapted to survive and successfully reproduce in their environments.
But natural selection cannot produce the perfect organism. Female shells have a low bulge along the edge of the shell that accommodates the enlarged gills in which it broods its larvae; males lack this bulge and are more streamlined. As with appearance, males that have the most attractive mating ritual potentially win the right to mate with the female. Birds provide the best examples of complex mating rituals, often involving dances, vocalizations, and fluffed up feathers.
Over time, the rituals most preferred by the females will become more common. There are no good examples of mating rituals among bivalves, but we can look to their relative, the octopus. The Day Octopus Octopus cyanea , found in the South Pacific and Indian Oceans, has an elaborate mating ritual that signals his mating intent to the female.
The ritual involves arm waving and skin darkening as the male approaches the female. If the male is positively received, the female is inseminated by the placement of spermatophores into her oviduct through the use of his modified arm. The female then lays thousands of eggs inside her lair. In this way, successful male rituals are passed down to offspring; males whose displays fail to attract a mate do not produce offspring.
Find more information about the Day Octopus here. Watch a video produced by North Carolina State University here. Freshwater pearl mussels have an interesting behavior of another sort related to reproduction. Female mussels brood their larvae, called glochidia Glochidium : The specialized larval form of freshwater pearl mussels that usually has hooks that enable it to attach itself to a host e.
But these glochidia depend on an intermediate host to complete their life cycle — they must attach to the gills or fins of a fish for a period of time before they can settle and metamorphose into baby mussels.
Some female freshwater mussels have modified flaps of tissue near their siphons Siphons : Posterior extensions usually two of the mantle through which water is directed in and out of the body, along with waste products and gametes of bivalves.
This type of selection often drives speciation. Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum. Imagine a population of mice living at the beach where there is light-colored sand interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, on the other hand, would not blend in with either the grass or the sand and, thus, would more probably be eaten by predators.
The result of this type of selection is increased genetic variance as the population becomes more diverse. Learning Objectives Contrast stabilizing selection, directional selection, and diversifying selection.
Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.
These types include the following:. If we look at a distribution of traits in the population, it is noticeable that a standard distribution is followed:. Example: For a plant, the plants that are very tall are exposed to more wind and are at risk of being blown over.
The plants that are very short fail to get enough sunlight to prosper. Therefore, the plants that are a middle height between the two get both enough sunlight and protection from the wind. This type of natural selection occurs when selective pressures are working in favour of one extreme of a trait.
Therefore when looking at a distribution of traits in a population, a graph tends to lean more to one side:. Example: Giraffes with the longest necks are able to reach more leaves to each. Selective pressures will work in the advantage of the longer neck giraffes and therefore the distribution of the trait within the population will shift towards the longer neck trait.
This type of natural selection occurs when selective pressures are working in favour of the two extremes and against the intermediate trait.
0コメント